The present invention relates to an apparatus for compensating for polarization mode dispersion (PMD) which is caused during transmission of a light wave via an optical waveguide, and to an optical communication network having such an apparatus.
The present invention also relates to a method for compensating for polarization mode dispersion (PMD) which is caused during transmission of a light wave via an optical waveguide.
In optical message transmission methods, light waves that are emitted from a transmitter are fed into an optical waveguide (LWL) and are transmitted via this optical waveguide (LWL) to a receiver. Optical waveguides may be composed, for example, of quartz or a specific quartz glass, or alternatively, they may be composed, for example, of normal glass or plastic. In “optical fibers” such as these, the “core” of the fiber generally has a refractive index nK, which is somewhat higher than the refractive index nM of the “cladding” which surrounds the core. This can be achieved, for example, by appropriate doping with other atoms.
The light which is used may, for example, be at a wavelength which is in the band between 1200 nm and 1800 nm.
In OTDM message transmission methods (OTDM=Optical Time Division Multiplexing), a signal which is transmitted via an optical waveguide, contains a number of signals (signal elements), to each of which one or more time slots is allocated.
If, by way of example, the respective signal (signal element) is intended to be used for transmitting a bit “1”, the respective transmitter transmits, for example, a so-called RZ pulse (RZ=Return to Zero) in the time slot allocated to the respective signal (signal element). If, instead of this, a bit “0” is intended to be transmitted, the respective transmitter does not feed any pulse into the optical waveguide in the corresponding time slot.
In optical communication networks, the light waves which are fed into the transmitter to the optical waveguide (LWL) are transmitted to the receiver via one or more network nodes, which are connected to one another via further optical waveguides. In the process, the interference caused by noise, crosstalk, delay time differences, etc., accumulates.
Optical regenerators, for example, so-called 3R regenerators, are used to compensate for the interference effects. A received optical signal is amplified, retimed and reshaped in a 3R regenerator (Reamplifying, Retiming, Reshaping) and is then passed on.
Particularly in the case of optical message transmission methods in which data is transmitted at high rates (10 Gbit/s or more), the path length which can be bridged without regenerators may be restricted by polarization mode dispersion (PMD). Polarization mode dispersion (PMD) may be caused, for example, by small asymmetries and by mechanical stresses in the core of the optical waveguide.
Polarization mode dispersion (PMD) leads to group delay times of different magnitude in the optical waveguide for different input polarization states. Signal distortion therefore may occur as a function of the respective input polarization.
The polarization mode dispersion behavior of an optical waveguide can be described approximately in the form of a model by a large number of birefringent elements connected in series whose optical (major) axes are, in each case, rotated with respect to one another.
Birefringent characteristics are provided, for example, by a large number of crystals, such as calcite, quartz, mica or tourmaline. Some substances may become birefringent when electrical fields are applied because the fields polarize the corresponding molecules.
In birefringent bodies, for example, crystals, the phase velocity of electromagnetic waves, for example, light waves, passing through them depends inter alia on the propagation direction of the waves with respect to the crystal axes and on the oscillation direction.
When they pass through birefringent bodies, light waves are generally split into two light wave bundles. The abovementioned optical axis is that direction in the body which is distinguished by the fact that the light waves which pass through the body parallel to it are not split.
An optical waveguide (or its approximate model in the form of the abovementioned birefringent elements connected in series) has two mutually perpendicular input polarization states (PSP=Principal States of Polarization), for which the output polarization states are, to a first approximation, independent of the wavelength of the respective light wave. The maximum differential group delay time (DGD) occurs between these two polarization states.
The two orthogonal PSP input polarization states, and the (maximum) differential group delay times, may vary with time owing to temperature changes, movements of the optical waveguide, etc. Thus, the signal distortion at the output of the optical waveguide likewise varies with time.
Differential group delay time compensators (PMDC=polarization mode dispersion compensator) are used to compensate for signal distortion occurring at the output of an optical waveguide. A differential group delay time compensator is intended to ensure that error-free data transmission is possible despite the polarization mode dispersion (PMD) which occurs during transmission of the light wave via the optical waveguide.
Conventional differential group delay time compensators have one or more PMD compensation devices, each of which has, for example, a polarization control device and a downstream birefringent element.
The light wave which emerges from the optical waveguide is supplied to the input of the differential group delay time compensator or to the (first) polarization control device in the (first) compensation device, and may then be supplied to further compensation devices. A (small) proportion of the light wave which emerges from the differential group delay time compensator or from the (final) compensation device is emitted at the output of the differential group delay time compensator, and is passed on to a PMD detection device.
The latter determines, for example, the quality of the light wave signal received, and hence, indirectly, the polarization mode dispersion (PMD) that has occurred in each case. As a reaction to this, appropriate control signals may be supplied to the polarization control device or devices, so that the polarization of the light wave passing through the respective polarization control device is changed appropriately. The light wave that emerges from the respective polarization control device is then supplied to the appropriate downstream birefringent element. Depending on the respective polarization state, as influenced by the polarization control device, the light waves are passed through the respective birefringent element with group delay times of different magnitude. This makes it possible to compensate (approximately) for the polarization mode dispersion (PMD) which has already occurred in the optical waveguide.
In the simplest case, a differential group delay time compensator has only a single PMD compensation device and only a single birefringent element. Only first-order polarization mode dispersion (PMD) can be compensated for in this case.
In order to compensate (approximately) for higher-order polarization mode dispersion (PMD) (that is to say, taking account of the wavelength dependency of the PSP and the differential group delay time (DGD)), the respective PMD compensation device must have, for example, a number of birefringent elements, connected in series, with polarization control devices connected between them, or a number of PMD compensation devices must be used.
An object of the present invention is to provide a novel apparatus for compensating for polarization mode dispersion (PMD) which is caused during transmission of a light wave via an optical waveguide, of providing a novel optical communication network having such an apparatus, and of providing a novel method for compensating for polarization mode dispersion (PMD) which is caused during transmission of a light wave via an optical waveguide.